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Preliminary results of a detailed study on the discharge probability for a triple-GEM detector at PSI G. Bencivenni, A. Cardini, P. de Simone, F. Murtas and D. Pinci The beam at M1 Davide Pinci, Cagliari University The positive beam was composed by protons and pions. By inserting 1 mm of aluminum on the beam line, protons loose energy more than pions and it’s possible to separate the two components of the beam after a magnetic dipole; By using the coincidence of two scintillator fingers we scanned the beam profile in order to find the pion and proton peak positions. In this configuration we centered our chambers on the pion peak. 17 cm The beam at M1: protons contamination A little contamination of protons was present at the + peak; Davide Pinci, Cagliari University By studying counting rate of a scintillator finger as a function of the discriminator threshold we estimate the ratio: p/tot=50 kHz/720 kHz 7% Protons with momentum of 350 MeV/c loose, by ionization, a mean energy 5 times higher than pions. Total rate Proton rate The beam at M1: the rate Davide Pinci, Cagliari University At low beam intensity, the rate has been measured by using a two scintillator finger coincidence (2x2 cm2). At high beam intensity we extrapolated the rate by using the GEM detector currents. 85 MHz on 2x2 cm2 Low beam-intensity The beam cross-section was 3x5 cm2 FWHM; The total rate was 300 MHz. High beam-intensity Discharges studies A discharge is mainly due to a streamer formation in a GEM hole which acts as a conductive channel between the two sides of the GEM causing a drop in the Vgem; A GEM recharge then occurs; Davide Pinci, Cagliari University The time for a GEM recharge is given by: total charge on the GEM ( 5 C) = 100 ms the current provided by the HV supply (50 A) The HV supply gives the average values of the monitored currents every 500 ms; A discharge is seen as an increase of the monitored current for a GEM electrode; On the pads a discharge in a GEM is seen as a drop of current because of the drop of the detector gain. The currents on the detector electrodes GEM 1 Single GEM discharges Davide Pinci, Cagliari University GEM 2 GEM 3 Pad current drop due to discharge Pad Beam Current discharge propagates The diffusion effect When the number of electrons in a hole becomes larger than the Raether limit (108) a streamer can occur; The electron diffusion in the transfer gaps can help to reduce the discharge probability by spreading the electron cloud; Davide Pinci, Cagliari University The more the transfer gap is wide the more the cloud is spread We built 3 detectors with different geometries using 10x10 cm2 Standard GEM: A: 3/1/2/1 the classical geometry; B: 3/1/7/1 big transfer gap before the 3rd GEM; C: 2/2/2/1 the same gap before any GEM; Lab test with alpha particles have shown a reduction by a factor 100 in discharge probability between chamber A and B. The gas mixtures studied Davide Pinci, Cagliari University We studied 3 different gas mixtures: Ar/CO2/CF4 60/20/20 : the classical one; Ar/CF4/C4H10 65/28/7: very good for time resolution (measured); Ar/CO2/CF4 45/15/40 : very promising for the time resolution (test beam is going on); Since the 1/nv term is the main contribution to the time resolution the Ar/CO2/CF4 45/15/40 gas mixture should give the same time performance as the Ar/CO2/C4H10 65/28/7. Drift field 3 kV/cm Results from the PSI test We performed a very high statistics study on the discharge probability; Each detector has integrated a total number of discharges as high as 5000; No apparent ageing or other damages have been observed on the 3 detectors (test is going on); Davide Pinci, Cagliari University Run 6 Run 43 Run 75 At the end of the test beam, after about 5000 discharges (also in very “hard” runs) the detectors work as in the first runs. Discharges in LHCb The area of GEM foils used in the final chambers in LHCb will be 20 x 24 cm2, but in that case the GEM foils will be segmented in 6 sectors of area 100 cm2; The sectors will be supplied through a resistor chain; Davide Pinci, Cagliari University Any damage in a sector won’t have effect on the other ones; Because of the particle rate in R1M1 (0.5 MHz/cm2) in order to have less than 5000 discharges/sector in 10 years discharge probability per incident particle < 10-12 Discharges: Ar/CO2/CF4 60/20/20 Inefficiency 1% due to recharge dead time Davide Pinci, Cagliari University Discharge probability < 10-12 Start of efficiency plateau: 99% in 25 ns per station. 1/nv = 2.25 ns the gain needed at the knee is 2.0 x 104 Narrow working region (10 20) Volts Discharges: Ar/CF4/C4H10 65/28/7 Inefficiency 1% due to recharge dead time Davide Pinci, Cagliari University Discharge probability < 10-12 Start of efficiency plateau: 99% in 25 ns per station. 1/nv = 1.7 ns the gain needed at the knee is 7.0 x 103 60 V wide working region Discharges: Ar/CO2/CF4 45/15/40 Since the 1/nv term for this gas mixture is the same of Isobutane-based one the efficiency knee is expected to be at the same gain value: 7 x 103 Vtot = 1250 V; Davide Pinci, Cagliari University Inefficiency 1% due to recharge dead time Discharge probability < 10-12 Start of efficiency plateau: 99% in 25 ns per station 60 V wide working region Davide Pinci, Cagliari University Conclusions 3 triple-GEM detectors have been tested with very high intensity hadron-beam (up to 300 MHz + with 7% of protons); About 5000 discharges have been integrated on each chamber without any damage or ageing effect; A discharge probability less than 10-12 per incident particle ensures safe operation for a GEM detector in R1M1; 3 set of data have been taken with 3 different gas mixtures: Ar/CO2/CF4 60/20/20 narrow working region 10 20 V; Ar/CF4/C4H10 65/28/7 wide working region 60 V; Ar/CO2/CF4 45/15/40 low discharge probability and very good time performance expected (test beam is going on); The new geometries with wide gap have shown a discharge probability of about one order of magnitude smaller. Wide gap chamber: alpha vs. pions The discharge probability suppression found in the wide-gap chamber with alpha particles (2 order of magnitude) has not been found also with penetrating particles (less than 1 order of magnitude). Why? We have an idea… Davide Pinci, Cagliari University Alpha particles don't penetrate behind the 1st GEM. The electron cloud is then amplified and diffused. A penetrating particle ionizes the gas all along the track. The statistical fluctuation of the ionization in a wide gap could increase significatively the charge density and a streamer can occur.